This invention relates to the control of the output of a loudspeaker, and relates to a loudspeaker circuit and signal processing method.
An important cause of loudspeaker failures is a mechanical defect that arises when the loudspeaker diaphragm is displaced beyond a certain limit, which is usually supplied by the manufacturer. Going beyond this displacement limit either damages the loudspeaker immediately, or can considerably reduce its expected life-time.
Furthermore, the combination of the use of small loudspeakers and the demand for high acoustic output results in the need for loudness maximisation methods, but increases the risk of exceeding the diaphragm displacement limit, also referred to as the ‘cone excursion’ limit. Small loudspeakers are also unable to efficiently reproduce low frequencies, so that a so-called loudspeaker linearisation method is often used to correct for this low efficiency. However, this also increases the risk of going beyond the displacement limit.
There exist several methods to limit the displacement of the diaphragm of a loudspeaker to provide loudspeaker protection.
One category consists of methods that process the input signal with variable cut-off filters (high-pass or other), the characteristics of which are controlled via a feedback loop. A displacement predictor is used to control the filters. Another category uses a feedforward strategy in which the signal is processed with a bank of band-pass filters with adjustable gains in such a way that excess excursion is prevented (by attenuating only the frequency bands that cause most excursion).
The use of a low frequency shelving and notch filter has also been proposed in a feedforward approach (in U.S. Pat. No. 7,372,966), which is controlled via the displacement predictor in a feedforward manner, using a model of the loudspeaker to pre-process the signal before sending it to the amplifier/loudspeaker.
U.S. Pat. No. 6,201,873 describes a setup for protecting against excess audio distortion (mainly caused by cone excursion), or driving the loudspeaker to its maximum (to maximum cone excursion). It uses a cone excursion transfer function (or a maximal voltage transfer function) module, the output of which is used to control a variable gain on the input. U.S. Pat. No. 6,201,873 suggests the use of a transfer function to give maximal voltage leading to just acceptable distortion, which is the maximal voltage that can be applied per frequency without causing excess distortion. The distortion being considered is that arising from excess cone displacement (no distortion in the converter and amplifier is considered).
This invention is based on the recognition that in many loudspeakers, the positive (outward-moving) and negative (inward-moving) diaphragm displacement peaks are not symmetrical when the loudspeaker is driven towards its displacement limit. In the following description, it is arbitrarily assumed that the negative diaphragm displacement is larger in absolute value than the positive displacement when the loudspeaker is driven towards its displacement limit, but the opposite may be true. This is often due to the asymmetry of the nonlinear behaviour of the loudspeaker with respect to the diaphragm displacement.
If the (measured or predicted) diaphragm displacement is limited by means of a protection scheme, it is likely that only the negative displacement peak is limited, while the positive peak never reaches the displacement limit. Thus, a protection scheme will prevent the maximum loudness being reached.